1. Field of the Invention
Our present invention relates to a process and an apparatus for the molding of shaped articles from a composite metallic refractory material.
The refractory composite materials which may be molded according to our invention comprise a complex matrix of a superalloy having a base of one or more ferrous metals, i.e. nickel and/or iron and/or cobalt, which contains chromium and a reinforcement phase constituted by monocarbide fibers of the transition metals. These composite materials are manufactured by the unidirectional solidification of an appropriate starting alloy. They are particularly useful in the manufacture of workpieces which are to be used at elevated temperatures, such as for example aeronautical turbine blades.
2. Description of the Prior Art
Refractory composite materials, such as described for example in commonly owned French Pat. No. 2,040,931 (see U.S. Pat. No. 3,871,835), are presently prepared in the form of slabs having simple geometrical shapes from which the more complex workpieces, such as the turbine blades mentioned hereinabove, are machined.
An attempt has been made to bypass the machining operation in order to directly obtain articles of complex shape by effecting the unidirectional solidification of the alloys in molds which directly impart the required dimensional precision and surface state to the molded pieces. Yet, such a procedure requires operating conditions which are extremely difficult to achieve. It is necessary, in effect, to obtain in all the straight sections of the article a structure, having columnar grains parallel to the direction of solidification of the material, in which each of the grains has a regular fibrous microstructure constituted by monocrystalline carbide fibers spaced from one another by about 10 microns. The diameter of the fibers is on the order of about 1 micron.
Since the growth of the grains is directed perpendicularly to the solidification front, and since the two phases of the composite crystallize simultaneously in each grain from the liquid growing perpendicularly to the solidification front, the solidification front must be rigorously planar. Moreover, in order to avoid the germination of parasitic grains forwardly of the solidification front as well as any cellular or dendritic-type disturbance of the growth of the composite material, the temperature gradient at the level of the solidification front must be of an elevated value, on the order of about 100.degree. to 200.degree. C./cm. In fact, the greater the solidification interval of the alloy and the greater the speed of the displacement of the solidification front, the greater ought to be the value of this gradient.
For this reason, the speed of displacement of the solidification front, which can be on the order of a fraction of a centimeter to several centimeters per hours, must be maintained at a constant value.
For workpieces of complex shape, in which the evolution of the shape of the section along the entire length of the workpiece entails a continuous variation of the surfaces that are in heat-exchanging relationship with the hot and cold sources, a continuous modification of the thermal flux at the level of the solidification front results. Satisfaction of all the conditions recited hereinabove has proven to be extremely difficult.
An apparatus for directional solidification with high thermal gradients has been described by G. J. May in the Journal of Physics E; scientific instruments, page 354, volume 8, May 1975. In this device the alloy meterial being heated is passed first through a heating zone and then into a cooling zone. The heating zone consists essentially of induction coils while the cooling zone is constituted by a liquid-metal bath. The device is, however, unsuitable for use in solidifying articles of complex shape by reason of the lack of planarity of the solidification front.